The present invention relates generally to the field of measurement methods and devices. More particularly, the present invention relates to the measurement of material properties using optically induced phonons.
Due to a growing need to the semiconductor and other industries to accurately measure properties (e.g., thickness, composition) of structures such as thin films, Impulsive Stimulated Thermal Scattering (ISTS) arose as a useful measurement technique. ISTS is described, for example, in U.S. Pat. Nos. 5,633,711 (entitled MEASUREMENTS OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS); 5,546,811 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES); and 5,812,261 (entitled METHOD AND DEVICE FOR MEASURING THE THICKNESS OF OPAQUE AND TRANSPARENT FILMS), the contents of which are herein incorporated by reference.
In measurement systems using ISTS, a pair of laser pulses overlap on the surface of a structure to form an optical interference pattern. The structure absorbs the interference pattern to initiate a response, such as a sound wave (e.g., an acoustic mode), that propagates in a plane of the structure. A second laser pulse or beam diffracts off the acoustic mode to generate a signal beam whose amplitude is modulated. A detector detects the signal beam to generate a signal waveform, which is then sent to a processor that processes the signal waveform to generate a Fourier Transform spectrum that includes a spectral feature (e.g., a Gaussian or Lorentzian peak) that corresponds to a frequency of the fundamental acoustic mode. The frequency of the peak relates to a physical property (e.g., thickness) of the sample.
The accuracy to which the frequency can be measured depends on properties of the signal waveform, as well as other aspects of the reflected and diffracted probe beam. For example, the measured frequency of the fundamental peak in the Fourier Transform spectrum is especially sensitive to parasitically scattered light from the probe beam. This can result in errors in the frequency measured during ISTS, thereby affecting the performance (e.g., repeatability and reproducibility) of the measurement. In addition, measurement of just the fundamental peak only allows one property (e.g., a single thickness) to be measured at a time. This, of course, is a disadvantage, as semiconductor devices typically include multiple thin films, and in these samples it is usually desirable to simultaneously measure the thickness of more than one film, or thickness and another property of interest (e.g., resistivity).
There thus exists in the art a need for systems and methods that can improve the performance and repeatability of thickness measurements, as well as to simultaneously measure more than one property of the sample at a time.
It is an object of the present invention to address the limitations of the conventional systems discussed above.
It is another object of the invention to provide improved methods and systems for measuring properties of samples, particularly those containing thin films and used in the semiconductor industry.
On aspect of the present invention is directed to an ISTS method that initiates, measures, and analyzes both the fundamental acoustic mode and higher-order acoustic modes of a sample under inspection.
Another aspect of the present invention relates to the ability to measure more than one property of a sample by measuring more than one mode of a Fourier transform spectrum simultaneously. Analysis of more than one mode can generate multiple property measurements simultaneously, for example, simultaneously measuring the thickness of two layers of a multilayer structure.
Another aspect of the invention relates to improved measurement repeatability resulting from measuring peaks corresponding to, e.g., a higher-order mode. Compared to the fundamental peak, this higher-order peak typically has a relatively narrow frequency bandwidth; it can therefore be measured with relative precision to improve properties such as measurement repeatability, reproducibility, and accuracy. In addition, the fundamental mode is affected adversely when light is scattered randomly off the surface of the sample. This light broadens the frequency bandwidth of the fundamental peak, thereby reducing the precision to which it can be measured. The frequency corresponding to a harmonic frequency (e.g., 2xcfx891) is less sensitive to parasitically scattered light than the fundamental frequency (e.g., xcfx891) and can therefore be measured with better precision.
Yet another aspect of the invention relates to measuring a frequency component of the Fourier transform spectrum, and a decay constant of the time-domain waveform. The decay constant can be related to the resistivity of the measured film, and thus this measurement yields both a thickness and resistivity of the measured sample. In other embodiments the decay constant can be related to the thickness of a second film in the sample.
One preferred embodiment of the present invention is directed to a method for determining properties of a multilayer structure. The method includes the steps of generating at least two excitation pulses, overlapping the two pulses to form an excitation pattern on or in the structure that modulates a probe beam to generate a signal beam, and detecting the modulation-induced signal beam. The signal includes at least two sub-component frequency values (e.g., a fundamental mode and a higher-order mode). The method also includes the step of analyzing the signal to determine at least two properties of the structure.
In a related embodiment, the signal includes a frequency component and a decay constant. The frequency component can be analyzed to determine a thickness of the structure, and the decay constant can be analyzed to determine a sructure""s resistivity.
Another embodiment of the present invention is directed to an apparatus for determining a property of a structure. The apparatus includes at least one source of excitation radiation; a measurement system that forms an excitation pattern on or in the structure, using the excitation radiation, that causes a modulation response by at least a portion of the structure; and a detector that detects a signal based upon the modulation response. The signal includes at least one frequency value and at least one harmonic value of the frequency value. Alternatively, the signal is characterized by at least one frequency value and at least one decay constant. The apparatus also includes an analyzer that analyzes the harmonic value or decay constant to determine a property of the structure.
These and other embodiments and aspects of the present invention are exemplified in the following detailed disclosure.